Default value to field if condition respected

I'm new in postgreSQl(still learning). I have 2 relations linked. 1st relation stores event and have a roomID attribute and the 2nd relation is for storing the capacity of the room(number of people it can hold-Stand and seat). Is there a way that when i add a room, its capacity is added in the 2nd relation? There are only 4 rooms (main room, side bar, metting room 1 and 2). Each has its capacity. what Im thinking is some kind of if Statement or switch which check which room has been inserted and add the int value.